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-   -   How to best set boundary conditions? (https://www.cfd-online.com/Forums/numeca/108389-how-best-set-boundary-conditions.html)

 DonQuijote October 22, 2012 11:38

How to best set boundary conditions?

Hello,

I'm working in FINE/turbo with a fan blade geometry, but I'm encountering a lot of problems in finding a converged solution.

I've reviewed all properties but I'm unabled to find a correct convergence, so I hope someone could help me. I think the problem should be related with boundary conditions.

I imposed a total pressure condition at inlet (since a don't have any other value), and a average static pressure outlet. Both of them are constant values since I don't have any kind of profile information (in fact, I don't know the static pressure outlet value). How important is to begin the computation with profiles??

Furthermore, thinking about the physics, if the fan has x rotational speed, it is supposed to obtain an y outlet pressure depending of it, so how can I pre-set this unknow initial condition?(So: known p. inlet, known rotational speed, unk. outlet)

I hope someone could enlighten to me...:confused:

 Hamidzoka October 23, 2012 00:35

Dear Sergio;
it is not neccessary to begin the computation with profiles and it has no negative effect on convergence. Profiles are just applied to find more realistic results specially when a validation with experimental data is required.
I think you should check the boundary conditions. They should be physically meaningful.
When flow passes thnrough the fan its total pressure increases.
So, you should consider this when setting the boundary conditions.
other possibility is the direction of rotation. check if you have correctly set the direction of rotation. It has a big effect on convergence.
what are your boundary conditions values? what is the level of RPM?

regards

 xyz33uu October 23, 2012 04:29

Hello,
Have U ever checked the message box that might show you the problems why U couldn't make a conerged solution. And show me, then maybe I could help.

Best Regards

 DonQuijote October 23, 2012 09:37

Thank you Hamidzoka and xyz33uu!

I'll explain better my case to show my main purpose and my problem. I'm studyng a specific fan blade (tunnel fans, not aircraft fan), so I have modelled it in autogrid, and now I want to validate the NUMECA results with information I have. After that, I want to change this blade and optimize it for another design point.

So, I have information about massflow, rpm and thrust. Then I set inlet condition as massflow imposed, and the outlet as averaged static pressure. With this configuration, the solution converges, but the axial thrust doesn't reach the real thrust data. The only value I can change is the outlet static pressure (as I unknow).

Thinking that, I was wondering how this parameter is caculated. Because I thought that axial thrust= (difference between inlet-outlet pressures)* A (Ainlet=Aoutlet). With this formula, I need a pressure difference of about 1500 Pa, but I'm unable to reach the mentioned thrust.

So any ideas that could help me?
Thank you very much for your interest!
Regards

 xyz33uu October 23, 2012 22:05

Maybe we have different understanding about thrust. In my opinion, thrust=H（head）*density*gravity*A=(（difference between inlet-outlet pressure）+density*（vout^2-vin^2）/2)*A

 DonQuijote October 24, 2012 03:22

:O I completely missed this part!!! Thank you john!!.

Then , if the thrust is T=massflow/g*V2+A*(P2-P1) (since the fan is static, V1=0), and I know all variables except P2 (P1=Patm), I can solve it, but when I use this parameter in Numeca, the resulting axial thrust is three times lower than the expected... Even when I consider a higher pressure outlet, the axial thrust is half the 'theoretical' one...(but, since the outlet pressure imposed in Numeca is static pressure, it has to be as Pamb.. is that correct?)

What I'm missing???

 xyz33uu October 24, 2012 06:18

Let's think about the unit of (massflow/g*v2),which is (Kg/s)/(m/s^2)*(m/s) that is equal to Kg. And that is different from the unit of thrust. So I think you have got a wrong equation, right?

 DonQuijote October 24, 2012 07:04

Independently of the ecuation (I use to obtain T in kg, changing for that the pressure units from N/m^2 to kg/m^2), I have a given 'theoretical' thrust (from technical data), and comparing it with the Numeca axial thrust, they are not corresponding (600 N to 1000 N).

But the problem converges quite well. I am sure about the rpm, and I've checked massflow, density, rpm in blade and hub area under the blade...

 xyz33uu October 24, 2012 08:40

I'm so sorry that I havn't notice you want to calculate the axial thrust. And thrust and axial thrust are different. Axial thrust is a component of thrust.

 DonQuijote October 24, 2012 09:10

But if the fan axis is aligned with the entire fan, it is not suposed to be the same? How would you calculate the component?

 xyz33uu October 26, 2012 06:31

Have you checked your private message box?

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